Metrology systems for providing position information regarding various structures are known. Metrology systems can be used to accurately measure the position of various points on a structure, so as to facilitate determination of the position, orientation, and/or shape of the structure.
For example, there is a need to know the position of line-of-sight communications and radar antennas. This need is particularly acute on spacecraft, where small deviations in the desired position of antenna elements and/or antenna supporting structures can substantially reduce the effectiveness of an antenna due to the large signal transmission distances involved. Typically, spacecraft must transmit and receive signals over distances of many thousands, sometimes millions, of miles. It is important for their antennas to be properly aligned and configured.
One contemporary method for determining the position of the elements of spacecraft antennas and the like to mount strain gauges thereon. The strain gauges are configured to detect movement of these elements away from their intended positions. In this manner, the orientation and shape of the antenna can be characterized.
However, as those skilled in the art will appreciate, such strain gauges are subject to variations in response due to temperature. Portions of a spacecraft may vary widely in temperature, as they are repeatedly exposed to the sun and then shaded from the sun. This is particularly true for satellites in geosynchronous orbit, where sunlight exposure can be substantial. Therefore, the use of strain gauges is not always an acceptable way to determine the position of spacecraft structures.
Other space metrology systems have been developed. For example, one system uses mechanically scanned laser radar to measure the distance to reflective targets arranged radially on a surface. Target position is determined by correlating the return signal and the scan angle. This method is capable of measuring multiple targets at a given scan angle as long as the targets are sufficiently spaced apart from one another, however this method lacks desired precision.
Another system uses a CCD-based device capable of tracking multiple illuminated targets (e.g., LEDs or reflectors) with sufficient precision for many space applications. The system is similar to currently-fielded star trackers. However this system is capable of making precision measurements in only two translational modes. This system does not provide range information.
As a result, there is a need in the art for a metrology system that is capable of the precise position measurement of a point on a structure in all three dimensions. Such a system should be substantially insensitive to the detrimental effects of sunlight, making it is suitable for spacecraft applications such as determining the position, orientation, and/or shape of antennas, other line-of-sight devices such as laser sources, and the like.